World’s fastest fiber-optic quantum key distribution at 1550nm
- Toward Practical Use of Quantum Cryptographic Technology for Ultimate Security -

The Photonics Research Institute (PRI) of the National Institute of Advanced Industrial Science and Technology (AIST), one of independent administrative institutions, has developed a thermoelectrically cooled single-photon detector boasting of fastest repetition rate (10MHz) in the world in the wavelength band for optical fiber communication, 1550nm (1nm=1/1,000,000,000m), based on the AIST-developed single-photon detecting technique, and succeeded in achieving the world’s highest key generation rate of 45kbit/s in a quantum key distribution experiment using 10.5km long optical fiber.

When secret information is to be shared between two parties separated by a long distance, cipher transmission is needed. It is possible to eavesdrop any (classical) communication, but it is rather difficult to crack coded message because of enormous time required for decoding. While the emergence of high speed computer or the development of new deciphering method would make it possible to break cipher, the use of quantum key distribution would ensure absolute secrecy.

The quantum key distribution consists in distributing a sequence of random binary numbers as a private key for encoding and decoding by sending a lot of photons and assigning 1-bit information to each photon, and it is characterized by capability of detecting wire-tapping. If two or more photons were assigned to 1 bit, the eavesdropper could make wire-tapping without being noticed by catching a photon to assess the bit value and sending the remainder to the receiver. Assigning exactly 1bit to a photon would make wire-tapping detected without fail. In this way, the quantum key distribution guarantees ultimate security in cipher communication.

The wavelength band at 1550nm is ideally suited for the quantum key distribution over a long distance owing to minimum transmission loss in optical fiber. When optical signals are sent over a distance of 100km at 1550nm wavelength, the transmission loss is as small as 20dB. On the other hand, the use of 800nm increases the loss to a despairing level of 300dB. It should be noted that even with 20dB loss at wavelength 1550nm, 99% of photons fail to reach the receiver with the key generation rate as low as 10bits/s or so. In the current cipher system, not using the quantum key distribution, the key generation rate is 100Mbits/s or so, however, long-lasting security cannot be expected owing to possible cipher breaking. In order to achieve ultimate security, it is essential to upgrade the key generation rate in quantum key distribution by reducing the transmission loss in optical fiber. It is also necessary to transmit and detect as many as possible photons in a short time span by reducing intervals of successive photons. The PRC-AIST has accomplished the fastest quantum key distribution in the world by upgrading the repetition rate of single-photon detector.

In the single-photon detection using an avalanche photodiode as an optical receiver, which is sensitive to 1550nm band of minimum transmission loss in optical fibers, a single-photon detector based on avalanche current amplification in the Geiger mode has been used up to now. However, when avalanche current is amplified, noises termed after-pulses rapidly grow at around repetition rate 1MHz. For this reason, it was rather difficult to realize repetition rate higher than 1MHz. The PRC-AIST developed an epoch-making single-photon detection technique without requiring avalanche current amplification. In this way, the occurrence of after-pulses caused by the avalanche current amplification in the Geiger mode has been effectively suppressed, and a single-photon detector of highest repetition rate 10MHz has been implemented (Photo). High-speed single-photon detection can be realized by changing the operational conditions to some extent without modifying electronic circuit used in the Geiger mode. Using such single-photon detectors and assigning 1-bit information to each photon, the highest key generation rate of 45kbits/s in the world has been achieved in the quantum key distribution at 1550nm over a 10.5km optical fiber.

Fore reference, see Japanese Journal of Applied Physics Part 2, vol. 43, no. 6A, pp. L735-L737 (2004).